MSC4190: Device management for application services

proposals/4190-as-device-management.md

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+# MSC4190: Device management for application services
+
+[MSC3202] allows application services to handle and send encrypted events.
+One part of [MSC3202] is the ability to masquerade devices using the `device_id`
+query parameter on C-S API requests, which eliminates the need to maintain
+individual access tokens for each application service user.
+
+However, application services don't have an endpoint to create devices for their
+users, which means that, in practice, encrypted application services still use
+`/login` with the `m.login.application_service` login type to create devices for
+their users.
+
+Consequently, such application services leave many unused but active access
+tokens for those users.
+
+Furthermore, if [MSC3861] were adopted, the `/login` endpoint would no longer be
+available for application services to use.
+
+This MSC proposes a dedicated API endpoint for application services to create
+and delete devices for users, addressing the existing gap to enable encrypted
+application services without `/login`.
+
+## Proposal
+
+This MSC proposes to extend existing endpoints to allow application services to
+create and delete devices for their users without relying on the `/login` and
+`/logout` mechanisms.
+
+As all changes here only apply to application services, guest access is not
+relevant.
+
+### **`PUT /_matrix/client/v3/devices/{deviceId}`**
+
+This endpoint is updated to allow the creation of a new device for a user, if
+the device ID does not exist. This behavior is only available to application
+services.
+
+This endpoint will use the 201 status code to indicate that a new device was
+created, in addition to the existing 200 status code for existing devices.
+
+The endpoint is rate limited. Servers may want to use login rate limits for
+device creation, although in most cases application services will disable all
+rate limits anyway.
+
+### **`DELETE /_matrix/client/v3/devices/{deviceId}`**
+
+This endpoint no longer requires User-Interactive Authentication for application services.
+
+### **`POST /_matrix/client/v3/delete_devices`**
+
+This endpoint no longer requires User-Interactive Authentication for application services.
+
+### **`POST /_matrix/client/v3/register`**
+
+Currently, the default behavior for `/register` is to create a new device and
+access token (i.e. login) in addition to creating the user. Similar to `/login`,
+creating an access token would no longer be possible with [MSC3861]. However,
+creating users via the endpoint is still required, so unlike `/login`, `/register`
+will not be removed entirely.
+
+Therefore, application services MUST call the endpoint with `inhibit_login=true`.
+Calls without the parameter, or with a different value than `true`, will return
+HTTP 400 with a new `M_APPSERVICE_LOGIN_UNSUPPORTED` error code.
+
+## Potential issues
+
+The change to `/v3/register` is technically backwards-incompatible, but it will
+break when switching to next-gen auth in any case, so a new endpoint version
+would not be useful.
+
+The endpoint could just stop returning access tokens to avoid breaking existing
+appservices that don't read that field, but an explicit error was chosen to
+avoid silent breakage of appservices that do depend on the field.
+
+## Security considerations
+
+This MSC lets application services delete devices without the usual
+re-authentication requirement. It is considered an acceptable risk, as
+application services have to be registered by the server admin.
+
+## Alternatives
+
+A new set of endpoints dedicated to application services could be added to the
+specification, like `GET|PUT|DELETE /_matrix/client/v3/appservices/{appId}/devices/{deviceId}`.
+
+This would have the advantage of not changing the behavior of existing endpoints.
+
+## Dependencies
+
+In order to use the devices created using this MSC, appservices need to be able
+to use device IDs as a part of identity assertion, as defined by [MSC4326].
+
+## Unstable prefix
+
+Until this MSC is stable, application services must opt-in to the new behavior
+by setting the `io.element.msc4190` flag to `true` in their registration file.
+
+[MSC4326]: https://github.com/matrix-org/matrix-spec-proposals/pull/4326
+[MSC3861]: https://github.com/matrix-org/matrix-spec-proposals/pull/3861